1. Field of the Invention
The present invention relates to a light emitting apparatus and a printer. The invention particularly relates to a light emitting apparatus including plural light emitting elements and a printer using the light emitting apparatus.
2. Description of the Related Art
A light emitting apparatus in which plural light emitting elements are arranged in a line manner or in a matrix is developed and commercialized for products like a display apparatus, a lighting apparatus, an exposure light source of an electrophotographic printer, or the like. The emitted light amounts of the light emitting elements are designed to be uniform in the apparatus including the plural light emitting elements.
In a display apparatus having a display surface where the light emitting elements are arranged in a matrix, when the light amounts, that is, luminances on the display surface are not uniform, a color irregularity occurs, or an image looks like having surface roughness.
In the electrophotographic printer including the exposure light source where the light emitting elements are arranged in a line manner, a luminance variation between the light emitting elements appears as a streak irregularity on a printed image and decreases a quality of the printed image.
A cause of the nonuniformity of the light amounts is not only a characteristic variation of the light emitting elements. A drive circuit configured to supply a drive current is attached to each of the light emitting elements. If characteristics of transistors included in the drive circuits are varied, currents are not uniform resulting in nonuniformity of the light amounts of the light emitting elements.
A drive circuit configured to compensate a variation of threshold voltages of the transistors is proposed in U.S. Pat. No. 6,229,506 (FIG. 3) and US2007/0063932 (FIGS. 1A, 4A, and 5A). The variation of the threshold voltages is absorbed in the drive circuit, and currents that do not rely on the threshold voltages are generated.
Even if the variation of the threshold voltages of the transistors is compensated, a variation of carrier mobility of semiconductor materials constituting the transistors would cause the current nonuniformity. In addition, even when all the current nonuniformity is removed by further improvement of the drive circuit, if light emitting efficiencies of the light emitting elements are not uniform, the variation in the light amount remains. When the light emitting elements are used as an exposure apparatus of the printer, a photosensitive member is irradiated with the emitted light through a lens array and optical nonuniformity of the lens array is added to the variation in the light amount.
As another method of eliminating the nonuniformity of the light amounts, it is conceivable to measure the light amounts of the light emitting elements and correct the image data input to the drive circuit on the basis of the measured light amounts. This method is called external correction since the correction is not performed within the drive circuit as described above. US Patent Application no. 2005/0190255 discloses an invention of the electrophotographic printer having an exposure head in which a deviation from a standard value of the light amounts of the light emitting elements is measured to correct a gradation signal using a correction table.
According to the external correction system, a correction parameter determined on the basis of the measured value of the light amount is stored in a memory for each light emitting element, and image data is corrected on the basis of this correction parameter.
The image data is normally transmitted from an external device such as a computer in a form of a digital signal. The image data is a signal for specifying the light amount of the light emitting element including gradation information for specifying an intermediate light amount in addition to turning-on and turning-off of the light.
The gradation signal is corrected in a correction circuit. The correction circuit is provided with a circuit configured to perform a correction computation or a look-up table in which a correction value is determined for each level of the gradation signal. The correction circuit corrects the input gradation signal to another digital gradation signal. The same number of the look-up tables are provided as the light emitting elements and saved in the memory.
The corrected gradation signal is converted into an analog voltage signal by a digital-to-analog converter (hereinafter, which will be referred to as DA converter). The DA converter is a circuit configured to decode the digital signal and output a voltage selected from a number of output voltages previously prepared. The number of the output voltages is the same as the number of the gradation levels.
The correction circuit and the DA converter are commonly provided to all the drive circuits and light emitting elements. Therefore, these circuits do not cause the nonuniformity of the light amounts. The nonuniformity of the light amounts caused by the drive circuits, the light emitting elements, and the lens array, which are located after the DA converter, could be cancelled by correcting and adjusting the voltage signal output from the DA converter.
If a variation range of the light amounts is wide, a range of the voltage signal must be wide enough to cancel the variation. That is, the corrected gradation signal is to generate a voltage signal in a wider range than the original gradation signal.
In addition, the voltage signal is to be finely adjusted to perform a correction at a high precision. Therefore, the gradation signal after the correction is to be a signal having more gradation levels than the original gradation signal.
To widen the gradation range and increase the number of gradation levels, the bit number of the gradation signal after the correction is higher than the bit number of the original gradation signal. However, a circuit scale in a correction computation circuit or the DA converter is at least doubled as the bit number of the digital signal to be dealt with is increased by only 1 bit. The circuit scale is enormously increased along with the increase in the bit number.